Non-symmetrical cathodes in cathode ray tubes



Sept. 2, 1958 H. s. ALLWINE 2,850,658

NON-SYMMETRICAL CATHODES IN CATHODE RAY TUBES Filed Feb. 21, 1956 INVENTOR. HHRBISUN S. HLLWINE' BY QZWQ Q United States PatentNON-SYNIMETRICAL CATHODES 1N CATHODE RAY TUBES Harrison S. Allwine,Trenton, N. 1., assignor to Radio Corporation of America, a corporationof Delaware Application February 21, 1956, Serial No. 566,929

7 Claims. (Cl. 31370) This invention relates to a multi-beam electrongun construction for cathode ray tubes. A typical such electron gun hasa plurality of cathodes, one for each electron beam. Each cathode isusually in the form of a hollow cylinder and contains a heater, one endof the cylinder being closed and coated with oxides to form an electronemitting surface. The cathodes are grouped side by side with theirelectron-emitting ends in a single plane. An apertured disk, the controlgrid, is positioned close to the emitter ends of these cathodes. It hasone aperture for each cathode, each aperture being much smaller than itscathode. Each aperture normally is positioned opposite the center of itsrespective cathode surface.

In some applications of electron guns of this type it is important tohave the electron beams originate from areas located close together. Theusual way of achieving close spacing is to position the several cathodescorrespondingly close together, at the same time keeping eachcorresponding grid aperture centrally aligned with its cathode. Whenthat is done, the diameters of the cathodes become a limiting factor indetermining how' close together the beam sources can be located. Inorder to have the beams originate still closer together, it isnecessary, in accordance with that practice, to use smaller diametercathodes so their centers can be closer to each other. It might appearthat nothing is sacrificed by thus using smaller size cathodes since theelectrons which go to make up a beam come almost entirely from thatportion of the cathode emitting surface which is substantially directlyopposite the control grid aperture. (Actually this portion may bedelineated as a slightly enlarged vertical projection of the apertureonto the cathode emitting surface. For convenience of reference, thisportion will be referred to as the primary area, and the rest of thecathode emitting surface will be referred to as the secondary area) Ithas been found, however, that actually the cathode life is considerablyreduced when much smaller cathodes are used, even though the gridapertures, the primary areas of the cathodes, and the other conditionsof use are all unchanged. It has been found, for instance, that the lifeof a 0.050 inch cathode is less than half as great as that of a standard0.120 inch cathode when both are used with 0.020 inch control gridapertures, and both are operated to give one milliampere continuous beamcurrent. i

I have found that I can provide closely spaced electron beams, and atthe same time so relate the parts as to achieve the longer cathode life.Heretofore, those skilled in the art have held to the tacit belief thatoptimum operational conditions were best obtained by using perfectlysymmetrical cathodes. When a secondary area was provided, it wasprovided symmetrically about the primary area. Therefore, when thesecondary area was reduced in an attempt to achieve closer beam spacing,it was reduced symmetrically, so that as spacing Patented Sept. 2, 19582 became closer and closer the amount of secondary area became less andless.

My invention consists in locating the grid apertures, and hence theprimary areas, close to an edge of their respecive cathodes; spacing thegrid apertures as close together as desired for close-spaced electronbeams; and at the same time departing from symmetry of cathode secondaryarea in such manner as to provide large secondary areas as a reservoirfor replacement barium for each cathode primary area. As such, a primaryarea either may or may not coincide with the centroidal axis of itscathode. An explanation of the principles involved in the operation ofmy invention can best be set forth in terms of theory as presentlyunderstood.

In normal use barium is slowly evaporated from the surface of a heatedoxide coated cathode, particularly in that area where electrons areactually being emitted. If this evaporation continues long enough, theresulting depletion of barium from the coating causes electron emissionto drop. However, barium is able to migrate slowly from one part of aheated cathode to another part. Therefore, it appears that the presenceof a secondary area, although not directly contributing to emission, canextend cathode life by supplying the barium atoms which migrate to theprimary area, there replacing those barium atoms which were lost throughevaporation. In this manner the secondary area acts as a bariumreservoir and enables the cathode to have a useful life much longer thanit would have if the cathode surface did not extend beyond the primaryarea.

Various embodiments of my invention as pictured in the drawings aredetailed below. In the drawings, Figure 1 shows a typical three-beamelectron gun 5 embodying my invention, the view being a perspective withparts broken away. The gun 5 is shown disposed in the cylindricalportion of a cathode ray tube envelope 6, and may include anaccelerating anode 7 and a focusing anode 8, in addition to a controlgrid 10 having three apertures 11, 12, and 13, and three separatecylindrical cathodes 15, 16, 17. The relative positioning of cathodesand control grid of such a gun is more clearly illustrated in Figure lawhich shows the control grid 10 with its three apertures 11, 12, and 13,and the three cathodes 15, 16, and 17, as viewed from the acceleratinganode side of the control grid. The apertures 11, 12, and 13 of thecontrol grid 10 are in triangular ar rangement close together so as toprovide closely spaced beams. The three cylindrical cathodes 15, 16, and17 are positioned parallel to each other behind the control grid with anelectron-emitting end of each of them opposite one of the gridapertures. in accordance with my invention, the cathodes are spacedclose together, though not actually in contact with each other. The gridapertures are opposite the cathode electron-emitting surfaces in thoseregions where they can be closest together, be symmetrically positionedwith respect to each other and to the cathode array, and still permitthe primary areas to fall fully upon the corresponding cathodes. Thus itwill be seen that the grid apertures are spaced closer to each otherthan are the longitudinal axes of the three cathodes but they are stillopposite their respective cathode surfaces.

Many other cathode shapes and arrangements within the scope of myinvention are possible. Figure 2 shows an embodiment having a controlgrid 26 with apertures 21, 22, and 23 in triangular arrangement butusing wedgeshaped cathodes 25, 26, and 27. Figure 3 shows an embodimentwith a control grid 30 having apertures 31, 32', and 33 and threecircularly cylindrical cathodes 35, 36, and 37 but rather than thetriangular arrangement these cathodes have an in-line arrangement whichis better adapted for some needs. In this embodiment the central cathode36- may be slightly smaller-than the two outer cathodes 35 and 37. Byhaving the central cathodes primary, area centered within itssurrounding area, a, more eflicien-tbarium reservoir aetion is achievedsince migration takes place from all directions toward the pri mary areaAs 'a result, the relative sizes of the three cathodes are made so thatthe life of the three cathodes are equal'toeach other. This designadjustment permits closer beam spacing than would otherwise be possiblein this embodiment, and at the same time equallzes cathode life.

Figure 4' shows another form for use when three inline beams aredesired. Here the beams are kept close together, the over-all diametersmall, and the total cathode areas large by extending the cathode endareas to 46 is nearly rectangular in cross-section, having two' slightlycurved sides; while the two outer cathodes 45 :and 47 have across-section shaped like a segment of a circle. 7 I V r The cathodesshown in Figures 2 and 4, though not i circularly cylindrical like thecathodes of Figures 1a and 3, are however, cylindrical in the broadsense of the Word.

In all cases relative positioning of cathode and grid aperture is suchthat at least some of the primary areas fall close to an edge of theircathodes and the electron beams are formed very close'to the centrallongtudinal axis of the cathode array. Yet, in each case there isprovided sufiicient cathode area to give long cathode life. Itshould benoted that While I'have described my invention as being incorporated inan electron gun having a single control grid common to all cathodes, itcould 7 also be applied to guns having a separate control grid for eachcathode or cathode primary area emitting surface.

J'Iclaim: V V

1. In a multi-beam cathode raygun, a plurality of cylindrical cathodesall having an end in a common plane;

and a control grid next adjacent said cathode ends and having acorresponding plurality of apertures, one aperturelocated opposite eachcathode end, a projection of each said grid aperture defining acorresponding cathode primary "area, each cathode end beingsubstantially V 7 larger than its primary area whereby to provide areservoir area for each cathode, the grid apertures being 'located toput the primary areas substantially at edges of the cathodes, thecathodes being located to put those V 'edgesclosely adjacent each other,and the reservoir areas adjacent to them having a correspondingplurality of a relatively much smaller apertures severally opposite thethree cathodes, said apertures being spaced closer to each otherthan'are the longitudinal axes of the three cathodes but still oppositetheir respective cathode surfacesJ 3. A three-beam electron gun for acathode ray tube comprising: a control grid having three aperturestherethrough positioned intriangular arrangement; and three cylindricalcathodes disposed parallel to each other, each having anelectron-emitting end arranged opposite and next adjacent to a difierentone of said apertures, and

5 each of said cathodes having its center positioned radially outward ofthe corresponding aperture center. 7

I 4. An electron gun for developing three separate electron beams,including three closely spaced circular cylindrical cathodes 'eachhaving an electron-emitting end, 10 said ends lying in a single planeand being spaced elec tricaily apart in triangular arrangement, and acontrol grid next adjacent ,said cathode ends having. three aperturestherethrough. in triangular arrangement and spaced such that each ofsaid aperturesis opposite a different one of said cathode ends and thatthe circledefined by.

the centers of said apertures is smaller than and con- 7 centric withthe circle defined by the centers .of. said cathode'ends.

' 5; A three-beam electron gun producing three electron beams which areside by side in a single plane, said gun including a control grid havingthree spaced apertures along a transverse line, three cylindricalcathodes dis:

posed with their separate longitudinal axes parallel'to each other andlying in a single plane, each of said cathodes having a relatively largeelectron-emitting end" opposite and next adjacent to a difierent' one ofsaid grid apertures, the central grid aperture being concentric with thecentral cathode, and the outer grid apertures being inward of thelongitudinal axes of' their corresponding;

cathodes.

6. A three-beam electron gun producing three electron beams which areside by side in a single plane, said gun including a control grid havingthree spaced small apertures along a transverse line, three circularlycylindrical cathodes disposed parallel to each other side by side alonga transverse line, each having a relatively large electron-emitting endopposite and next adjacent to a different one of said grid apertures,the central grid aper- I tore being concentric with the central cathodewhereby optimum barium reservoir effect is achieved, the outer aperturesbeing opposite aportion of the end surface near the inner edge of theircorresponding cathodes 7 whereby minimum beam spacing for a given sizeof cen tral cathode is achieved, the outer cathodes being larger throughpositioned in triangular arrangement and spaced 7 such that each'of saidapertures is opposite a diflferent one of said cathode ends closer tothe apex of said end than to its curved edge. 7 i

References Cited in the file of this patent V V UNITED STATES PATENTS2,661,436 Van Ornier Dec. 1, 1953 l 2,735,031 Woodbridge Feb. 14, r956 r2,747,134 'Allwine May 22, 1956 than the central cathode whereby tocompensate in size next adjacent said ends and having three aperturesthereaw-A...

